Aerosol Microphysical and Radiative Effects on Continental Cloud Ensembles

Aerosol–cloud–radiation interactions represent one of the largest uncertainties in the current climate assessment. Much of the complexity arises from the non-monotonic responses of clouds, precipitation and radiative fluxes to aerosol perturbations under various meteorological conditions. In this study, an aerosol-aware WRF model is used to investigate the microphysical and radiative effects of aerosols in three weather systems during the March 2000 Cloud Intensive Observational Period campaign at the US Southern Great Plains. Three simulated cloud ensembles include a low-pressure deep convective cloud system, a collection of less-precipitating stratus and shallow cumulus, and a cold frontal passage. The WRF simulations are evaluated by several ground-based measurements. The microphysical properties of cloud hydrometeors, such as their mass and number concentrations, generally show monotonic trends as a function of cloud condensation nuclei concentrations.Aerosol radiative effects do not influence the trends of cloud microphysics, except for the stratus and shallow cumulus cases where aerosol semi-direct effects are identified. The precipitation changes by aerosols vary with the cloud types and their evolving stages, with a prominent aerosol invigoration effect and associated enhanced precipitation from the convective sources. The simulated aerosol direct effect suppresses precipitation in all three cases but does not overturn the aerosol indirect effect. Cloud fraction exhibits much smaller sensitivity(typically less than 2%) to aerosol perturbations, and the responses vary with aerosol concentrations and cloud regimes. The surface shortwave radiation shows a monotonic decrease by increasing aerosols, while the magnitude of the decrease depends on the cloud type.     

基于CloudSat/CALIPSO卫星资料的青藏高原云辐射及降水的研究进展

青藏高原上空的云及其相关联的降水和辐射影响了高原上空非绝热加热的空间结构。2006年卫星发射升空的CloudSat/CALIPSO卫星提供了定量的、完整的云垂直结构信息。本文回顾了国内外基于该资料进行的青藏高原上云宏观和微观结构特征,云与降水相关性,云辐射效应以及模式中的云-辐射问题方面的研究。指出抬升的青藏高原上水汽较少,限制了高原上云的垂直高度,对云层厚度和层数有显著压缩作用。在云量及其季节变化上,单层云的相对贡献大于亚洲季风区的其他区域;夏季对流云比较浅薄,积云发生频率最高,云内滴谱较宽;降水云以积云和卷云为主,云对总降水的贡献随着云层数增多而减小,降水增强时高层冰粒子的密集度趋于紧密;夏季青藏高原地区云的净辐射效应在8 km高度存在一个厚度仅1 km左右但较强的辐射冷却层,而在其下（4~7 km高度之间）为强的辐射加热层。最后展望了未来需要进一步开展的研究。     

The radiative influence of aerosol effects on liquid-phase cumulus and stratiform clouds based on sensitivity studies with two climate models

Aerosol effects on warm (liquid-phase) cumulus cloud systems may have a strong radiative influence via suppression of precipitation in convective systems. A consequence of this suppression of precipitation is increased liquid water available for large-scale stratiform clouds, through detrainment, that in turn affect their precipitation efficiency. The nature of this influence on radiation, however, is dependent on both the treatment of convective condensate and the aerosol distribution. Here, we examine these issues with two climate models—CSIRO and GISS, which treat detrained condensate differently. Aerosol–cloud interactions in warm stratiform and cumulus clouds (via cloud droplet formation and autoconversion) are treated similarly in both models. The influence of aerosol–cumulus cloud interactions on precipitation and radiation are examined via simulations with present-day and pre-industrial aerosol emissions. Sensitivity tests are also conducted to examine changes to climate due to changes in cumulus cloud droplet number (N _c); the main connection between aerosols and cumulus cloud microphysics. Results indicate that the CSIRO GCM is quite sensitive to changes in aerosol concentrations such that an increase in aerosols increases N _c, cloud cover, total liquid water path (LWP) and reduces total precipitation and net cloud radiative forcings. On the other hand, the radiative fluxes in the GISS GCM appear to have minimal changes despite an increase in aerosols and N _c. These differences between the two models—reduced total LWP in the GISS GCM for increased aerosols, opposite to that seen in CSIRO—appear to be more sensitive to the detrainment of convective condensate, rather than to changes in N _c. If aerosols suppress convective precipitation as noted in some observationally based studies (but not currently treated in most climate models), the consequence of this change in LWP suggests that: (1) the aerosol indirect effect (calculated as changes to net cloud radiative forcing from anthropogenic aerosols) may be higher than previously calculated or (2) lower than previously calculated. Observational constrains on these results are difficult to obtain and hence, until realistic cumulus-scale updrafts are implemented in models, the logic of detraining non-precipitating condensate at appropriate levels based on updrafts and its effects on radiation, will remain an uncertainty.     

An algorithm for generating stochastic cloud fields from radar profile statistics

An algorithm is described for generating stochastic three-dimensional (3D) cloud fields from time–height fields derived from vertically pointing radar. This model is designed to generate cloud fields that match the statistics of the input fields as closely as possible. The major assumptions of the algorithm are that the statistics of the fields are translationally invariant in the horizontal and independent of horizontal direction; however, the statistics do depend on height. The algorithm outputs 2D or 3D stochastic fields of liquid water content (LWC) and (optionally) effective radius. The algorithm is a generalization of the Fourier filtering methods often used for stochastic cloud models. The Fourier filtering procedure generates Gaussian stochastic fields from a “Gaussian” cross-correlation matrix, which is a function of a pair of heights and the horizontal distance (or “lag”). The Gaussian fields are nonlinearly transformed to give the correct LWC histogram for each height. The “Gaussian” cross-correlation matrix is specially chosen so that, after the nonlinear transformation, the cross-correlation matrix of the cloud mask fields approximately matches that derived from the input LWC fields. The cloud mask correlation function is chosen because the clear/cloud boundaries are thought to be important for 3D radiative transfer effects in cumulus.The stochastic cloud generation algorithm is tested with 3 months of boundary layer cumulus cloud data from an 8.6-mm wavelength radar on the island of Nauru. Winds from a 915-MHz wind profiler are used to convert the radar fields from time to horizontal distance. Tests are performed comparing the statistics of 744 radar-derived input fields to the statistics of 100 2D and 3D stochastic output fields. The single-point statistics as a function of height agree nearly perfectly. The input and stochastic cloud mask cross-correlation matrices agree fairly well. The cloud fractions agree to within 0.005 (the total cloud fraction is 18%). The cumulative distributions of optical depth, cloud thickness, cloud width, and intercloud gap length agree reasonably well. In the future, this stochastic cloud field generation algorithm will be used to study domain-averaged 3D radiative transfer effects in cumulus clouds.     

湖南秋季积层混合云系飞机人工增雨作业方法

统计分析2007—2016年秋季湖南省长沙市地面气象观测资料、湖南省飞机人工增雨作业资料, 得到湖南省秋季积层混合云系的降水分布情况、一般结构特征和相应的飞机增雨作业方法。使用多普勒天气雷达、GRAPES_CAMS数值模式和中小尺度气象站网等资料对典型作业天气过程进行云降水物理和数值模拟分析, 采用成对对流云和基于TREC算法的回波跟踪等方法进行作业效果评估。归纳得到湖南省秋季积层混合云系人工增雨作业条件判别的12个宏微观指标, 探讨在使用运7飞机、碘化银烟条作业装备条件下, 开展飞机增雨作业的最佳催化时机、部位和剂量。针对积层混合云系中的降水性层状云系、积云对流泡, 飞机增雨适宜作业的区域、播撒高度和催化剂量:在过冷高层云的-15~-5℃层, 播撒达到30 L-1的人工冰晶浓度; 在过冷积云的-15~-7℃层, 静力催化使冰晶浓度达到30 L-1或动力催化达到100 L-1。这些方法在实践中取得了较好的人工增雨作业效果。     

中国区域气溶胶对东亚夏季风的可能影响（I）：硫酸盐气溶胶的影响

利用NCAR的新一代GCM CAM3.0模式离线耦合一个气溶胶同化系统,模拟研究了中国区域硫酸盐气溶胶的直接气候效应对东亚夏季风及其降水的影响。结果显示：中国区域硫酸盐气溶胶引起全球平均的直接辐射强迫为-0.25 W/m²,中国内陆约25°N以北普遍降温,而海表温度升高。由此导致海陆温差缩小,东亚夏季风强度减弱,中国地区季风降水明显减少,而尤以积云降水减少起主要作用。硫酸盐气溶胶对中国地区的对流活动起抑制作用。     

A quantitative performance assessment of cloud regimes in climate models

Differences in the radiative feedback from clouds account for much of the variation in climate sensitivity amongst General Circulation Models (GCMs). Therefore metrics of model performance which are demonstrated to be relevant to the cloud response to climate change form an important contribution to the overall evaluation of GCMs. In this paper we demonstrate an alternative method for assigning model data to observed cloud regimes obtained from clustering histograms of cloud amount in joint cloud optical depth—cloud top pressure classes. The method removes some of the subjectivity that exists in previous GCM cloud clustering studies. We apply the method to ten GCMs submitted to the Cloud Feedback Model Intercomparison Project (CFMIP), evaluate the simulated cloud regimes and analyse the climate change response in the context of these regimes. We also propose two cloud regime metrics, one of which is specifically targeted at assessing GCMs for the purpose of obtaining the global cloud radiative response to climate change. Most of the global variance in the cloud radiative response between GCMs is due to low clouds, with 47% arising from the stratocumulus regime and 18% due to the regime characterised by clouds undergoing transition from stratocumulus to cumulus. This result is found to be dominated by two structurally similar GCMs. The shallow cumulus regime, though widespread, has a smaller contribution and reduces the variance. For the stratocumulus and transition regimes, part of the variance results from a large model spread in the radiative properties of the regime in the control simulation. Comparison with observations reveals a systematic bias for both the stratocumulus and transition regimes to be overly reflective. If this bias was corrected with all other aspects of the response unchanged, the variance in the low cloud response would reduce. The response of some regimes with high cloud tops differ between the GCMs. These regimes are simulated too infrequently in a few of the models. If the frequency in the control simulation were more realistic and changes within the regimes were unaltered, the variance in the cloud radiative response from high-top clouds would increase. As a result, use of observations of the mean present-day cloud regimes suggests that whilst improvements in the simulation of the cloud regimes would impact the climate sensitivity, the inter-model variance may not reduce. When the cloud regime metric is calculated for the GCMs analysed here, only one model is on average consistent with observations within their uncertainty (and even this model is not consistent with the observations for all regimes), indicating scope for improvement in the simulation of cloud regimes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

中国区域气溶胶对东亚夏季风的可能影响（I）：硫酸盐气溶胶的影响

 利用NCAR的新一代GCM CAM3.0模式离线耦合一个气溶胶同化系统,模拟研究了中国区域硫酸盐气溶胶的直接气候效应对东亚夏季风及其降水的影响。结果显示：中国区域硫酸盐气溶胶引起全球平均的直接辐射强迫为-0.25 W/m²,中国内陆约25°N以北普遍降温,而海表温度升高。由此导致海陆温差缩小,东亚夏季风强度减弱,中国地区季风降水明显减少,而尤以积云降水减少起主要作用。硫酸盐气溶胶对中国地区的对流活动起抑制作用。     

Idealized simulation of the Colorado hailstorm case: comparison of bulk and detailed microphysics

One of the purposes of the Fourth Cloud Modeling Workshop was to compare different microphysical treatments. In this paper, the results of a widely used bulk treatment and five versions of a detailed microphysical model are presented. Sensitivity analysis was made to investigate the effect of bulk parametrization, ice initiation technique, CCN concentration and collision efficiency of rimed ice crystal–drop collision. The results show that: (i) The mixing ratios of different species of hydrometeors calculated by bulk and one of the detailed models show some similarity. However, the processes of hail/graupel formation are different in the bulk and the detailed models. (ii) Using different ice initiation in the detailed models' different processes became important in the hail and graupel formation. (iii) In the case of higher CCN concentration, the mixing ratio of liquid water, hail and graupel were more sensitive to the value of collision efficiency of rimed ice crystal–drop collision. (iv) The Bergeron–Findeisen process does not work in the updraft core of a convective cloud. The vapor content was always over water saturation; moreover, the supersaturation gradually increased after the appearance of precipitation ice particles.     

Precipitation responses to radiative effects of ice clouds: A cloud-resolving modeling study of a pre-summer torrential precipitation event

The precipitation responses to the radiative effects of ice clouds are investigated through analysis of five-day and horizontally averaged data from 2D cumulus ensemble model experiments of a pre-summer torrential precipitation event. The exclusion of the radiative effects of ice clouds lowered the precipitation rate through a substantial reduction in the decrease of hydrometeors when the radiative effects of water clouds were switched on, whereas it increased the precipitation rate through hydrometeor change from an increase to a decrease when the radiative effects of ice clouds were turned off. The weakened hydrometeor decrease was associated with the enhanced longwave radiative cooling mainly through the decreases in the melting of non-precipitating ice to non-precipitating water. The hydrometeor change from an increase to a decrease corresponded to the strengthened longwave radiative cooling in the upper troposphere through the weakened collection of non-precipitating water by precipitation water.     

The role of shallow convection in the water and energy cycles of the atmosphere

The Canadian Centre for Climate Modelling and Analysis atmospheric general circulation model (AGCM4) is used to study the role of shallow convection in the hydrologic and energy cycles of the atmosphere. Sensitivity tests with AGCM4 show a marked effect of the parameterization of shallow convection in the model. In particular, including the parameterization of shallow convection produces considerably enhanced vertical mixing and decreased stratiform cloud amounts in the lower subtropical atmosphere over the oceans. The differences in simulated stratiform cloud amounts are associated with a change in the globally averaged outgoing shortwave radiative flux at the top of the atmosphere of about 11 W m⁻². Additionally, precipitation rates are considerably reduced for stratiform clouds and enhanced for convective clouds in the subtropics, if the parameterization of shallow convection is included in the model. Additional tests show that the simulated responses in cloud amounts and precipitation to the treatment of shallow convection are robust. Additional simulations with modified closures for deep convection and other changes to the treatment of convection in the model still lead to similar responses of the model results.     

对流云对大气气溶胶和相对湿度变化响应的数值模拟

利用二维面对称分档云模式研究了气溶胶颗粒物浓度和尺度谱分布对混合相对流云微物理过程和降水的影响, 并重点讨论了气溶胶效应随环境相对湿度的变化。结果表明, 在初始热力和动力条件相同的情况下, 相对清洁的海洋性云在发展和成熟阶段能更有效地产生雨滴、 冰晶和霰粒, 形成更强的雷达反射率。随着气溶胶浓度增加, 比如在本文模拟的污染大陆性云中, 气溶胶粒子数浓度的增加限制云滴增长, 不利于降水粒子的形成。模拟结果也发现, 环境相对湿度对气溶胶效应有显著影响, 即当地面相对湿度从50%增大到70%时, 所模拟的云从浅对流泡发展为深对流云; 气溶胶对云微物理特性和降水的影响在干空气中较小, 但在湿空气中表现非常显著, 这与前人结果一致。随着相对湿度的增加, 冰相粒子出现的时间提前, 增长加快, 云砧范围扩大, 但相对来说, 降水起始时间对相对湿度的变化比气溶胶更敏感。     

Evaluation of cloud and precipitation parameterization using a single-column model: A TWP-ICE case study

Cloud and precipitation parameterization schemes are evaluated, and their sensitivity to the method and/or parameters used to determine cloud physical processes is examined using a singlecolumn version of the Unified Model (SCUM). In the experiment for TWP-ICE, cloud fraction is overestimated (underestimated) in the upper (lower) troposphere due to the wet (dry) bias. The precipitation rate is well simulated during the active monsoon period, but overestimated during the suppressed monsoon and clear skies periods. In the moist convection scheme, trigger condition and entrainment process affect the lower tropospheric humidity through the impact on convective occurrence frequency and intensity, respectively. Strengthening the trigger condition and using the adaptive entrainment method alleviate the low-level dry bias. In the microphysics scheme, more large-scale precipitation is produced with prognostic rain, due to rain sedimentation considering vertical velocity of rain drop, than with diagnostic rain. Less ice/snow deposition with the prognostic two-ice category results in lower ice water content and upper-level cloud fraction than with the diagnostic splitting method for the twoice category. In the cloud macrophysics scheme, the prognostic cloud fraction and cloud/ice water content scheme produces a larger cloud fraction and more cloud/ice water content than the diagnostic scheme, mainly due to detrainment from moist convection (cloud source) that surpasses the effect of convective heating and drying (cloud sink). This affects temperature by influencing the radiative, convective, and microphysical processes. The experiment with combined modifications in cloud and precipitation schemes shows that interaction between modified moist convection and cloud macrophysics schemes results in more alleviation of the cold bias not only at the lower levels but also at the upper levels.     

层状云对积云发展和降水的影响——一种云与云之间影响的数值模拟

利用我们已建立的二维积云降水模式,在相同的大气层结条件下,模拟了孤立积云和层状云中积云的发展和降水情况。结果表明,层状云的存在对积云的发展有显著的促进作用,降水量可加大到几到几十倍,从而认为积层混合云系可能是产生大雨和暴雨的一种重要机构,这与梅雨锋里锋区混合云系常产生暴雨的观测事实比较符合。     

华北地区一次气溶胶与浅积云微物理特性的飞机观测研究

2014年8月15日,山西省人工降雨防雹办公室在山西忻州开展了气溶胶和浅积云的飞机观测,本文利用机载云物理资料,详细分析了华北地区气溶胶、云凝结核（CCN）和浅积云微物理特性及其相互影响。主要结论有:（1）此次过程的边界层高度约为3600 m,不同层结情况下,0.1～3 μm尺度范围内的气溶胶粒子浓度N_a、有效直径D_a和CCN数浓度的垂直廓线明显不同,近地面N_a可达2500 cm?3。（2）CCN的主要来源为积聚模态、爱根模态或者核模态的气溶胶颗粒,0.2%过饱和度下,气溶胶活化率（AR）在各高度层的结果变化不大;0.4%过饱和度下,AR随着高度增加而降低。（3）后向轨迹模式分析表明,2 km以下的气溶胶主要来自于当地城市排放,由细颗粒污染物组成;2 km以上的气溶胶主要来源于中国西北和蒙古地区的沙漠,由亚微米沙尘组成,溶解度相对较低,可作为潜在的冰核。（4）本文细致分析了两块相邻浅积云（Cu-1和Cu-2）的云物理特性。Cu-1云底高度约4500 m,云厚约600 m,云体松散,夹卷较多;云中液态含水量（LWC）基本保持在0.5 g m?3,云粒子浓度N_c平均值为278.3 cm?3,云滴有效直径D_c整体在15 μm以内;毛毛雨滴粒子浓度最大值为0.002 cm?3,云中几乎无降水粒子;粒子谱宽随着高度增加而增大,主要集中在30 μm以内。Cu-2云底高度约3900 m,云厚约1200 m,云体密实;云中过冷水丰沛,LWC有多个超过1 g m?3的区域,云顶附近出现冰晶,云中粒子从凝结增长状态直接进入到混合相态;积云内部粒子水平分布不均,同一高度N_c相差较大,最大可达1240 cm?3。D_c随着高度增加而增大;粒子谱宽随着高度增加而拓展,最大可达1100 μm,谱型由单峰向多峰转变;降水粒子和冰晶图像大多为霰粒子、针状和板状。     

南方夏旱期积云含水量和降水效率的云模式估算

在胡志晋二维对流云模式的物理框架上增加了对整块积云的含水量、地面降水量、降水效率的估算部分.改进后的模式模拟了福建夏旱期37 个降水个例,并估算出南方夏旱期冷云、混合云和暖云的含水量、地面降水、降水效率,其结果与湖南积云的实测值较接近.分析了含水量在关键时段的主要分布情况,为研究南方夏旱期积云的人工影响方法提供物理依据.     

Applying the WRF Double-Moment Six-Class Microphysics Scheme in the GRAPES_Meso Model: A Case Study

This study incorporated the Weather Research and Forecasting (WRF) model double-moment 6-class (WDM6) microphysics scheme into the mesoscale version of the Global/Regional Assimilation and PrEdiction System (GRAPES_Meso). A rainfall event that occurred during 3–5 June 2015 around Beijing was simulated by using the WDM6, the WRF single-moment 6-class scheme (WSM6), and the NCEP 5-class scheme, respectively. The results show that both the distribution and magnitude of the rainfall simulated with WDM6 were more consistent with the observation. Compared with WDM6, WSM6 simulated larger cloud liquid water content, which provided more water vapor for graupel growth, leading to increased precipitation in the cold-rain processes. For areas with the warmrain processes, the sensitivity experiments using WDM6 showed that an increase in cloud condensation nuclei (CCN) number concentration led to enhanced CCN activation ratio and larger cloud droplet number concentration (N_c) but decreased cloud droplet effective diameter. The formation of more small-size cloud droplets resulted in a decrease in raindrop number concentration (N_r), inhibiting the warm-rain processes, thus gradually decreasing the amount of precipitation. For areas mainly with the cold-rain processes, the overall amount of precipitation increased; however, it gradually decreased when the CCN number concentration reached a certain magnitude. Hence, the effect of CCN number concentration on precipitation exhibits significant differences in different rainfall areas of the same precipitation event.     

沙尘气溶胶对云和降水影响的模拟研究

采用二维分档云模式,对比背景大气气溶胶分布,讨论了扬沙和沙尘暴天气条件下矿物气溶胶对云微物理结构、光学特性以及降水形成的影响.结果表明:扬沙和沙尘暴天气增加大气中大核和巨核的浓度,促进云中水汽的活化,使降水提前出现,暖云和冷云降水量均大幅增加,但可忽略巨核增加对云光学厚度和反照率的作用;当矿物沙尘粒子同时作为有效的云凝结核和冰核参与云的发展时,冰核浓度增加使水成物有效半径减小,抑制了暖云和冷云降水,云内存留的大量冰晶增强云的光学厚度和反照率.     

一次东北冷涡云降水垂直结构特征分析

利用辽宁阜新国家站(121.7458°E,42.0672°N)的毫米波云雷达(8 mm)和微雨雷达(12.5 mm)对2020年8月12-13日东北冷涡影响下的一次降水过程进行了观测,分析了云降水的垂直结构特征并探讨了降水机制。结果表明：本次过程中,云水平方向发展不均匀,以层状云和层积混合云为主,云内有时还嵌有对流泡。云降水阶段性变化明显,先后出现了层状云降水、层积混合云降水和对流云降水。层状云降水和层积混合云降水均表现出明显的亮带特征,但层积混合云降水的雷达回波强度、回波顶高和降水强度明显大于层状云降水。对流云降水的雷达回波会因强降水而产生明显衰减,因此回波顶高不能表示出实际的云顶情况。层状云降水阶段,云雷达反射率随高度降低增长缓慢,雨滴在下落过程中受蒸发和碰并的共同作用,反射率降低。与层状云降水相比,层积混合云降水的碰并效应强,且由于前期降水对近地面的增湿作用,使云下蒸发弱。对流云降水阶段,反射率的增长主要发生在冰水混合层,有利于大滴的产生,拓宽了云滴谱,提高了碰并效率。     

华南黑碳气溶胶浓度与南海夏季风关系的年代际突变

利用重建的华南区域黑碳气溶胶(Black Carbon, BC)浓度资料,分析其与南海夏季风在年际尺度上的关系。结果表明,华南区域BC浓度与南海夏季风的关系在2000年前后有明显的突变,由显著负相关变为显著正相关,即由高BC浓度弱季风变为高BC浓度强季风。通过合成对比分析,发现1988—1999年(第一时间段)的华南BC主要气候效应是间接辐射强迫作用:华南BC使云粒子半径减小,抑制华南区域春季降水,增加了云的生命期,从而使到达地面的短波辐射减少,表面和低层大气降温。负温度异常激发了异常反气旋,在南海区域即有东风异常。到夏季,东风异常减弱了季风强度,同时抑制了南海地区的降水。2000—2010年(第二时间段)的华南BC主要气候效应是直接辐射强迫作用:春季高BC浓度通过直接气候效应,增暖大气,加强降水,但是雨日减少,从而使到达地面的短波辐射增多,表面和低层大气增温。正温度异常激发了异常气旋,在南海区域即有西风异常一直维持到夏季,增大了季风强度,同时增强了南海地区的降水。